geometic series of n terms

A geometric series of n terms can be wiriiten:
a + ar + ar^2 + ar^3 +... + ar^n-1 where a, is the first and r is the common ratio, how would you put this in sigma notation? How can a formula be written in a, r and n for the sum of this series?
Thank you!
Keith Stevens

A geometric series of n terms can be wiriiten:
a + ar + ar^2 + ar^3 +... + ar^n-1 where a, is the first and r is the common ratio, how would you put this in sigma notation? How can a formula be written in a, r and n for the sum of this series?
Thank you!
Keith Stevens

Observe that if we multiply the series by we get all the terms but the first and last cancelling: